The present invention relates to the electroless plating art. Specifically, a method is provided for controlling the replenishment rate of a plating solution constituent chemical component during electroless plating.
Large multi-layer circuit boards are formed by electroless plating copper on circuit traces on the circuit board. In order to maintain the quality of the copper deposit, the rate of copper deposition should be maintained substantially constant. Prior art techniques for controlling the rate of copper deposition include controlling a number of chemical parameters such as temperature, pH, copper concentrations, cyanide concentration and formaldehyde concentration. At intervals of approximately one hour, a determination of plating rate is made by measuring the weight gain of a copper coupon suspended in the plating bath. The operator uses the weight gain measurement to adjust the set point of a formaldehyde controller in order to obtain a desired plating rate. The time between rate measurement and the difficulty of manually determining the right formaldehyde concentration to achieve a given plating rate results in uncertain quality control over the plating deposition rate, and ultimately the quality of the metallic deposit.